1. Field of the Invention
The present invention relates to a content transmission system, a communication device and a content transmission method.
2. Description of the Related Art
Recently, a home network has been proposed in which, by establishing an internet protocol (IP) connection respectively for image processing devices within a household, content data can be shared between a plurality of image processing devices. In the proposed home network, a technology to ensure copyright protection is digital transmission content protection (DTCP).
DTCP applies round-trip time (RTT) technology and, by setting a tolerance value for the round-trip time (RTT time) between two devices included in the home network, restricts the distance between the two devices at which they can transmit and receive content data. However, in actuality, a response time includes a variety of delays due to network conditions, and if the response time tolerance value is too low, the communication between the two household devices may be restricted. On the other hand, if the response time tolerance value is increased, content data is acquired from the home network by a device that exists externally to the home network, and it is thus difficult to meet the objectives of copyright protection. For example, with DTCP, if the response time tolerance value is restricted to 7 ms, taking into account the delays of a wireless router or the like, in theory, the maximum range of access is 1050 km at the speed of light.
Also, a wireless communication device is disclosed in Japanese Patent No. 3669293 that performs data communication after mutual authentication. More specifically, the above wireless communication device performs mutual authentication by reducing the transmission power of the wireless signal in comparison to that at the time of data communication. With this set up, as the distance between the wireless communication device and the authentication device is restricted, the number of devices that are able to transmit and receive content data with the wireless communication device is also restricted.